The Downing Street papers reveal that two-thirds of Margaret Thatcher's first cabinet opposed buying the US Trident missile system and the chiefs of staff were not unanimous. But this did not stop the "Iron Lady" from going ahead with the deal behind their backs.

The cabinet were only told of the decision to buy the original Trident C-4 system when the details of the secret deal with Jimmy Carter leaked out in the US.

The 1981 cabinet papers show that John Biffen, the trade secretary, privately warned Thatcher in March not to underestimate the electoral damage the anti-nuclear movement could inflict. He correctly predicted that the Campaign for Nuclear Disarmament was going to be bigger than its 1950s Aldermaston heyday.

The first women's peace camp was set up at Greenham Common that September and in the same month more than 250,000 people joined a national CND rally.

Disclosure of the scale of cabinet opposition is revealed in a note from a 10 February Downing Street meeting at which only John Nott, her defence secretary, and Lord Carrington, her foreign secretary, were present. Nott told Thatcher a full debate on nuclear defence policy was essential "since two-thirds of the party and two-thirds of the cabinet were opposed to the procurement of Trident. Even the chiefs of staff were not unanimous."

Nott, who supported the decision, went on to say he believed Britain would have to buy five rather than the four submarines envisaged and the bill would double to £10bn. Carrington harboured no such doubts: "Failure to acquire Trident would have left the French as the only nuclear power in Europe. This would be intolerable."

If Thatcher had any qualms about not telling the cabinet about her secret deal with Carter in July 1980 to buy Trident, her cabinet secretary, Sir Robert Armstrong, was quick to reassure her. In a handwritten "secret and personal" note he told her that when Harold Macmillan negotiated the deal in Nassau with John Kennedy to buy Polaris, the cabinet "ratified the decision and the agreement, but played no part in arriving at the original decision or in laying down the negotiating brief".

In a separate note he reminded her that the Labour prime minister, Harold Wilson, did not consult the cabinet in 1974 when he ordered the Chevaline missile system. Thatcher's biographer Hugo Young said Britain's possession of an independent nuclear deterrent was the aspect of her inheritance about which she countenanced least argument.